Drilling wells in oil and gas reservoirs is a lengthy operation, in some cases, lasting over a month for a single well. In addition, drilling, for example offshore, is performed in difficult and hazardous conditions; it mobilizes equipment that is complex and expensive. To reduce the cost associated with the exploration or exploitation of oil and gas reservoirs, drillers are pressed to drill wells faster and increase the yield of the drilling equipment. In some places, increasing the rate of penetration (or “ROP”) of a drill string through the reservoir and reducing the frequency at which the drill string is removed from the well to replace a worn drill bit have become a necessity to make oil or gas extraction economical.
Several components of a drill string may be optimized to achieve a higher ROP. The drill bit is one of these components: improving its aggressivity, its stability, as well as its durability has significant impact on the speed at which wells may be drilled. Examples of efforts to improve drill bit performances are shown in U.S. Pat. Nos. 5,551,522, 5,582,261, 5,549,171. Further, certain types of rocks may present unique challenges for improving drill bit performance. Examples of efforts to improve drill bit performances in plastic shales, one of the rocks encountered near or in oil and gas reservoirs, are described in U.S. Pat. No. 6,564,886.
Methods of designing drill bits for increased performance may involve estimating the trajectory of cutter elements mounted on a rotating drill bit, as well as the bottom hole pattern (or “BHP”) resulting from this trajectory. In cases where the drill bit and the wellbore axis are essentially collocated, sometimes referred to as concentric drilling cases, the BHP is relatively easy to estimate. In contrast, in cases where the drill bit and wellbore axis are tilted and/or offset, sometimes referred to as eccentric drilling cases, the BHP is more difficult to estimate. An example method of designing drill bits that involves estimating the BHP is shown in U.S. Pat. No. 7,844,426 and one that accounts for drilling in eccentric mode in U.S. Pat. No. 7,860,693. More recently, U.S. Pat. App. Pub. No. 2012/0046869 discloses methods that rely on mathematical approximations of the trajectory of the cutters in the eccentric mode to estimate the BHP. In the current state of the art, few methods account for drilling in the eccentric mode, and the performance of drill bits operating at least partially in eccentric mode, typically drill bits mounted on a rotatable bent motor, remains mediocre.